The tightest ultracool dwarf binary system ever observed

Astrophysicists discover the closest and oldest ultracool dwarf binary ever observed.


Previously, astronomers had only detected three short-period ultracool dwarf binary systems. They were relatively young- up to 40 million years old. In a recent study, astrophysicists at Northwestern University and the University of California San Diego (UC San Diego) have discovered an extreme system: the tightest ultracool dwarf binary system ever observed.

This newly discovered system is known as LP 413-53AB. It consists of a pair of ultracool dwarfs. The system is estimated to be billions of years old. Surprisingly, its orbital period is at least three times shorter than all ultracool dwarf binaries discovered so far.

The proximity between the two stars is like this: they take less than one Earth day to revolve around each other. Each star’s “year” lasts just 17 hours.

Chih-Chun “Dino” Hsu, a Northwestern astrophysicist who led the study, said, “It’s exciting to discover such an extreme system. We knew these systems should exist in principle, but no such systems had been identified yet.”

The system was initially discovered on January 10 as part of a session on Stars and Their Activity. The team found this while exploring archival data.

Hsu created an algorithm that uses a star’s spectral data to simulate it. Astrophysicists can ascertain a star’s chemical makeup, temperature, gravity, and rotation by analyzing the light the star emits. This analysis demonstrates the star’s radial velocity, or the rate it moves towards and away from the observer.

Hsu found an oddity while looking at the LP 413-53AB’s spectral data. Hsu thought there was just one star in the system because early observations caught it when the stars were closely aligned and their spectral lines overlapped. But, as the stars migrated in their orbit, the spectral lines separated into pairs and underwent opposite-direction shifts. Hsu found there were two stars entangled in a close binary.

Hsu used powerful telescopes at the W.M. Keck Observatory to observe the phenomenon for himself. On March 13, 2022, the team turned the telescopes toward the constellation Taurus, where the binary system is located, and observed it for two hours. Then, they followed up with more observations in July, October, and December, as well as January 2023.

Professor Adam Burgasser said, “When making this measurement, we could see things changing over a few minutes of observation. Most binaries we follow have orbit periods of years. So, you get a measurement every few months. Then, after a while, you can piece together the puzzle. With this system, we could see the spectral lines moving apart in real-time. It’s amazing to see something happen on a human time scale in the universe.”

“The observations confirmed what Hsu’s model predicted. The distance between the two stars is about 1% of the distance between the Earth and the sun. This is remarkable because when they were young, something like 1 million years old, these stars would have been on top of each other.”

The team thinks that the stars either migrated toward each other as they evolved or could have come together after the ejection of a third — now lost — stellar member. More observations are needed to test these ideas.

Hsu also said that “by studying similar star systems, researchers can learn more about potentially habitable planets beyond Earth. Ultracool dwarfs are much fainter and dimmer than the sun, so any worlds with liquid water on their surfaces — a crucial ingredient to form and sustain life — need to be much closer to the star. However, for LP 413-53AB, the habitable zone distance is the same as the stellar orbit, making it impossible to form habitable planets in this system.”

“These ultracool dwarfs are neighbors of our sun. To identify potentially habitable hosts, it’s helpful to start with our nearby neighbors. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found.”

Journal Reference:

  1. Chih-Chun Hsu, Adam Burgasser and Christopher A. Theissen. Discovery of the Exceptionally Short Period Ultracool Dwarf Binary LP 413-53AB. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/acba8c